Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Tread compositions typically contain one or more elastomers and one or more types of reinforcing materials such as particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of the desired properties. Many of the polymers used in the manufacture of vulcanizates such as, e.g., tire components, are elastomeric. In addition to natural rubber, some of the most commonly employed include high-cis polybutadiene, often made by processes employing catalysts, and substantially random styrene/butadiene inter-polymers, often made by processes employing anionic initiators.
Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of filler particles can be improved by increasing their interaction with the elastomer(s) and/or decreasing their interaction with each other. Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof. Terminal chemical modification often occurs by reaction of a terminally active polymer with a functional terminating agent. However, chemical modifications that can be undertaken with carbanionic polymers often do not work for polymers made via Ziegler-Natta catalytic processes and, at least to some extent, vice versa.